1. Field of the Invention
This invention is in the field of heat treatment of finely divided particles, such as raw cement meal and seeks to improve the heat economy of such a system by providing a rapid heating for the meal in the clinker stage, and separately collecting coarse and fine grained fractions from the treatment.
2. Description of the Prior Art
The dry burning method is predominantly employed in industry for the manufacture of cement, particularly portland cement. Originally, the entire process of preheating, deacidification and sintering was performed in a rotary tubular kiln. Subsequently, a separate heat exchanger outside the rotary tubular kiln was provided to accomplish the preheating and finally, the deacidification reaction with its relatively great heat consumption was transferred into a separate calciner. Use of separate stages has the advantage that both the preheating and the calcination were significantly more efficient because of better heat transfer with the fine grained material in suspension in hot gases rather than in a product bed. Consequently, the specific heat consumption in the burning of cement particles could be considerably reduced.
Approximately 30 to 40% of the overall fuel charge is used in the sintering burning of the material in the rotary tubular kiln when the pretreatment steps are separated as stated, and where there is complete deacidification in the calciner. The term "sintering burning" in this context refers to heating the product from approximately 900.degree. C. following calcination up to a temperature in the range of incipient alite formation at approximately 1250.degree. C. as well as to the range of the sintering reaction of lime and silicon components above 1300.degree. C. into technical tricalciumsilicate characterized by the formation of melt phases. The clinker reaction or the "maturing" is considered concluded when all but slight residues of CaO are consumed by means of the reaction with Ca.sub.2 SiO.sub.4 (belite).
It was early recognized, as in German Pat. No. 337 312 of May 1921, that a quick heating resulting from intimate contact between flame and product had to be provided in order to achieve a good burning result not only during the sintering but in the heating-up phase between deacidification and alite formation. It was then proposed that a subdivided rotary kiln be used, having a sintering part rotating slower than the other part so that the kiln which serves to heat up material to the immediate proximity of the sintering point rotates with such a high speed that the product is lifted to the immediate proximity of the apex of the kiln and falls freely from there through the cross section of the kiln. Significantly higher heat transfer should occur in the convection zone theoretically as a result. This proposal, however, was never adopted because of the enormous amounts of dust which arose in the system gas.
The perception that it is very advantageous for reasons of energy saving and reaction kinetics to perform the heating between the calcination of the raw material and the maturing with the highest possible speed has led to other proposals. For example, a so-called fast burning of raw material is mentioned in DL Letters Pat. No. 97 409 where it is proposed to quickly heat a powdery or agglomerated mixture in an eddy type layer in a reactor, followed by sintering up to maturation. When extremely high heating gradients are employed in the temperature range between approximately 1100.degree. and 1350.degree. C., a reduction of the maturing time by approximately 70% can be achieved. For example, the reactor can be reduced in size or its throughput increased. The maturation may occur at lower temperatures with standard dwell times. The extremely high heating gradients also tend to avoid deactivation of the calcined product.
The attainment of fast burning, accordingly, produces a noticeable reduction of energy consumption which in turn leads to reduced capital and operating costs.
While there have been unresolved difficulties created by attempts to heat the raw material quickly, fast heating does have an effect on the properties of the matured cement clinker. For one, the clinker is not as highly sintered, i.e., it becomes more porous. A considerably higher amount of dust is produced in the exhaust gases of the rotary tubular kiln as a result. On the other hand, a porous clinker is desirable from the standpoint of being easier to grind into powder.
In a standard burning installation wherein there is countercurrent flow of burning product and gases, a porous clinker has a harmful effect as a result of the increased dust concentration since dust circulations are formed which result in considerable reduction of system performance and an increase in the specific energy consumption. In such instances, the proportion of additives which form melt phases have to be increased in order to reduce the dust circulation. Fluxes such as expensive iron ore have been used for this purpose. This has the disadvantage that the calcining temperature and consequently the specific fuel requirement is increased and the outlay for grinding energy is raised as a result of the production of a very hard clinker. Both factors have a negative effect regarding higher production costs and reduced output.